Essay On Transport Across The Rest Of The Renal Tubule Notes

Reabsorption across the rest of the kidney

Thin descending limb

i) Very low permeability to NaCl, finite urea permeability-secretes urea into the tubule lumen

-The high interstitial concentrations of NaCl, urea provide the osmotic energy for passively reabsorbing water which secondarily concentrates NaCl in the lumen of the thin descending limb of henle

ii)Water: high permeability to water, high expression of AQP1

1) Thin ascending limb

i) Na+ : Sodium transport by the thin descending and thin ascending limbs of Henle’s loop is entirely passive and paracellular- paucity of mitachondira

Urea: as tubule fluid reaches tip of the loop of Henle, urea concentration is higher in the medullary interstitium than in the lumen, so the deepest portion of thin descending loop of Henle secretes urea through facilitated diffusion through urea transporter UT-A2

iii) water: impermeable to water

2) Thick ascending limb

i) Na+

Transcellular: 50%

-Na/K/CC cotransporter couples the inward movement of 1 Na+, 1K+, 2Cl- in electroneutral process driven by downhill concentration gradients of Na+ and Cl-. The K+ recycles to lumen through apical K+ channels and replenish luminal K+ Furoesemide-loop diuretic, inhibits Na/K/CL cotransport

-NHE3

-Paracellular: Lumen positive voltage- provides the driving force for the diffusion of Na+ across the tight junctions

ii) Cl-: Na/K/CC cotransporter in the apical membrane, the Cl- exits through the basolateral cell membrane through Cl- channels-100% transcellular

iii) Water: TAL and all downstream segments have low water permeability in the absence of ADH, allows nephron the generate a low luminal Na+ concentration, low osmolarity. This allows the distal nephron to reabsorb water passively from a hypo-osmotic luminal fluid into isomotic blood when ADH increases water permeability

iv) HC03-: Reabsorbs small amount of bicrabonate

Ca 2+ : TAL reabsorbs 25% of the filtered calcium, half of the calcium is reabsorbed in the TAL passively by a paracellular route, driven by the lumen positive voltage. ADH which makes the transepithelial voltage more positive-increase Calcium reabsorption

3) Distal convoluted tubule

i) Na+ : Mostly transcellular, apical Na+ uptake is mediated by electroneutral Na/Cl cotransporter: Blocked by thiazide diuretics

ii) Cl-: Na.CL co transporter

iii) Water: impermeable but permeability...

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